WEAR ELSEVIER wear251(2001)1452-1458 Temperature effects on the tribological behavior of alumina reinforced with unidirectionally oriented SiC whiskers D-S. Lim a,C,*D-S Park.B-D. Han.T-S Kan C Ho Jang a Division of Material Science Engineering, Korea University, Seoul 136-701, South Korec Ceramic Materials Group, Korea Institute Machinery Materials, Changwon, South Korea Ceramic Processing Research Center, Hanyang University, Seoul 136-701, South Korea Abstract In the present study, the influence of temperature on the tribological behavior of alumina reinforced with Sic whisker addition up to 20 vol. is presented. Wear tests were performed at selected temperatures with alumina composites prepared by a modified tape casting method Above a 673 K wear test temperature, the wear and frictional characteristics behaved differently compared with tribological behavior at 403 K. The results of this study indicate that as the temperature increased, the reinforcement effect of the unidirectionally oriented SiC whisker increased and the grain size effect was decreased. The effect of whisker orientation on friction and wear characteristics at the higher temperature was not dependent on test temperatures. The highest wear rate was obtained with a Sic whisker oriented parallel to the tape casting direction at all tested temperatures. The friction and wear mechanism of alumina composites reinforced with SiC at a high temperature is discussed based on SEM observations of won surfaces and energy dispersion X-ray analysis. C 2001 Elsevier Science B.V. Keywords: Alumina, SiC whiskers; High temperature; Tribology 1. Introduction the matrix grain size may be important. Although some researchers investigated high temperature friction and wear Silicon carbide whiskers have been incorporated into of alumina-SiC whisker composites, it is not clear what the ramic materials for improving their fracture toughness [1-31. most important microstructural variable is. Yust reported Improved fracture toughness allows the silicon carbide that both the friction coefficient and the wear rate of Sic whisker reinforced alumina ceramic composite to be used whisker reinforced alumina composite increased as the test for cutting nickel-based super alloys. Although researches temperature increased up to 673K, then decreased with have concentrated on improving the fracture toughness of further increases in temperature [4]. According to Yust, the the composite, its practical applications often need better tribochemical reaction product on the surface is responsible tribological properties including a low friction coefficient for the variation in friction and wear of the composite. Del- and wear rate. The friction and wear of ceramic materials is laCorte reported that the self-mated alumina-SiC whisker a complicated topic and involves several variables. One of composite exhibited a lower wear rate than that of monolithic the variables is the microstructure of the ceramic material. alumina against itself 5]. While most of the previous studies Since the silicon carbide whiskers affect the mechanical were performed with the composite with randomly oriented properties of the composite, they certainly influence its silicon carbide whiskers, our previous report was carried tribological properties. According to our previous report out with a composite of aligned silicon carbide whiskers the matrix grain size exerted a strong influence upon wear [4]. Due to the highly anisotropic microstructures,wear of alumina-SiC whisker composite at room temperature of the composite varied according to the sliding direction, [4]. Meanwhile, applications of the ceramic materials often with respect to the whisker or fiber alignment direction [7] involve wear resistance at elevated temperatures, as in the his study, alumina ceramics reinforced with the metal forming industry. At elevated temperatures, the situ- aligned silicon carbide whiskers were tested against silicon ation changes and the microstructural variables other than nitride balls at high temperatures in order to investigate high temperature wear mechanisms of the whisker reinforced Corresponding author. Tel: +82-2-3290-3272: fax: +82-2-928-3584 composites and the effect of orientation of those whiskers E-mail address. dslim a mail. korea. ac kr(D-S. Lim). on the friction and wear of the composite 0043-1648/01/S -see front matter e 2001 Elsevier Science B v. All rights reserved P:S0043-1648(01)00784-0
Wear 251 (2001) 1452–1458 Temperature effects on the tribological behavior of alumina reinforced with unidirectionally oriented SiC whiskers D.-S. Lima,c,∗, D.-S. Park b, B.-D. Han b, T.-S. Kan a,c, Ho Jang a a Division of Material Science & Engineering, Korea University, Seoul 136-701, South Korea b Ceramic Materials Group, Korea Institute Machinery & Materials, Changwon, South Korea c Ceramic Processing Research Center, Hanyang University, Seoul 136-701, South Korea Abstract In the present study, the influence of temperature on the tribological behavior of alumina reinforced with SiC whisker addition up to 20 vol.% is presented. Wear tests were performed at selected temperatures with alumina composites prepared by a modified tape casting method. Above a 673 K wear test temperature, the wear and frictional characteristics behaved differently compared with tribological behavior at 403 K. The results of this study indicate that as the temperature increased, the reinforcement effect of the unidirectionally oriented SiC whisker increased and the grain size effect was decreased. The effect of whisker orientation on friction and wear characteristics at the higher temperature was not dependent on test temperatures. The highest wear rate was obtained with a SiC whisker oriented parallel to the tape casting direction at all tested temperatures. The friction and wear mechanism of alumina composites reinforced with SiC at a high temperature is discussed based on SEM observations of worn surfaces and energy dispersion X-ray analysis. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Alumina; SiC whiskers; High temperature; Tribology 1. Introduction Silicon carbide whiskers have been incorporated into ceramic materials for improving their fracture toughness [1–3]. Improved fracture toughness allows the silicon carbide whisker reinforced alumina ceramic composite to be used for cutting nickel-based super alloys. Although researches have concentrated on improving the fracture toughness of the composite, its practical applications often need better tribological properties including a low friction coefficient and wear rate. The friction and wear of ceramic materials is a complicated topic and involves several variables. One of the variables is the microstructure of the ceramic material. Since the silicon carbide whiskers affect the mechanical properties of the composite, they certainly influence its tribological properties. According to our previous report, the matrix grain size exerted a strong influence upon wear of alumina–SiC whisker composite at room temperature [4]. Meanwhile, applications of the ceramic materials often involve wear resistance at elevated temperatures, as in the metal forming industry. At elevated temperatures, the situation changes and the microstructural variables other than ∗ Corresponding author. Tel.: +82-2-3290-3272; fax: +82-2-928-3584. E-mail address: dslim@mail.korea.ac.kr (D.-S. Lim). the matrix grain size may be important. Although some researchers investigated high temperature friction and wear of alumina–SiC whisker composites, it is not clear what the most important microstructural variable is. Yust reported that both the friction coefficient and the wear rate of SiC whisker reinforced alumina composite increased as the test temperature increased up to 673 K, then decreased with further increases in temperature [4]. According to Yust, the tribochemical reaction product on the surface is responsible for the variation in friction and wear of the composite. DellaCorte reported that the self-mated alumina–SiC whisker composite exhibited a lower wear rate than that of monolithic alumina against itself [5]. While most of the previous studies were performed with the composite with randomly oriented silicon carbide whiskers, our previous report was carried out with a composite of aligned silicon carbide whiskers [4]. Due to the highly anisotropic microstructures, wear of the composite varied according to the sliding direction, with respect to the whisker or fiber alignment direction [7]. In this study, alumina ceramics reinforced with the aligned silicon carbide whiskers were tested against silicon nitride balls at high temperatures in order to investigate high temperature wear mechanisms of the whisker reinforced composites and the effect of orientation of those whiskers on the friction and wear of the composite. 0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0043-1648(01)00784-0
D.-S. Lim et al./Wear251(2001)l452-1458 Table I orma Starting ceramic powder compositions of samples(vol % tape casting direction Component Parallel with 995 79.5 AKP-30, Sumitomo Chemical, Osaka, Japan. bEP, Junsei Chemical, Tokyo, Japan. SCW#1, Tateho Chemical Industries, Hyogo, Japan. Normal with 2. Experimenta Fig. 2. Schematic repr ion of the sliding direction with to the tape casting and lamination direction. whiskers are well Table I shows the compositions of samples used for according to these directions this study. For sample A, the two ceramic powders were mixed by planetary ball milling for 8h. Ethanol, alumina lel, normal with tape casting direction, and normal with the balls 5mm in diameter(SSA999, Nikkato Corp, Tokyo, lamination direction). Fig. 2 shows a schematic illustration Japan) and a plastic jar were used for mixing. The mixed of the sliding directions with respect to whisker orientation powder was hot pressed at 1823K for I h under 30 MPa. The wear volume of the flat sample was obtained by wear For samples T10 and T20, a modified tape casting method track length multiplied cross-sectional worn area that was was employed for aligning the whiskers. The diameters measured from the center portion of the groove by a surface and lengths of the Sic whiskers used in this study were profilometer after cleaning in a ultrasonic bath. Each wear 1-1. 4 and 10-20 um, respectively, according to information test was repeated three times and the average values of the supplied from the manufacturer. Details of the processing measurements were expressed in the results. The wear scar Tape cast products were cut and laminated at 353 K under with an energy dispersive X-ray analyzer: Scope equipped conditions and modification are described elsewhere [6, 8] was examined by a scanning electron micre 50 MPa for 0.5h. Binder burn-out was performed at 823 K for 10 h in open air, and then the sample was hot pressed at 2123K under 30 MPa for 1 h 3. Results Tribological behavior was studied by using the ball-on- reciprocating flat geometry. Fig. I shows a schematic dia- iding on sample A was carried out on the surface in a gram of the high temperature wear tester. A silicon nitride normal direction with hot pressing direction, Different slid- ball(NDB200, Norton Co., Northboro, MA, USA) of ing directions with respect to the whisker orientation are 35 mm in diameter was used against a flat alumina-Sic shown in Fig. 2. Fig. 3 shows the values of the average whisker composite sample surface which was polished with friction coefficient and standard deviations measured dur- um diamond slurry. The normal load was 40N and the sliding parallel with tape casting direction for the test average speed was 10 mm/s. The reciprocating stroke was materials at different temperatures. The friction coefficient 5.64 mm and the duration was I h. Wear tests were per- formed at a range of temperature between 403 and 873K. 0.7- In order to examine the effect of whisker orientation on wear rate, tests were carried out on three directions(paral 0.5 Load cell heater c0.3 Ball DC Moto Computer Indicator Whisker content(vol % Fig. 3. Friction coefficients between the alumina-SiC whisker composite Fig. 1. Schematic diagram of the high temperature tester. and silicon nitride ball as a function of whisker content
D.-S. Lim et al. / Wear 251 (2001) 1452–1458 1453 Table 1 Starting ceramic powder compositions of samples (vol.%) Component Sample A T10 T20 Al2O3 a 99.5 89.5 79.5 MgOb (wt.%) 0.5 0.5 0.5 SiC whiskerc – 10 20 a AKP-30, Sumitomo Chemical, Osaka, Japan. b EP, Junsei Chemical, Tokyo, Japan. c SCW#1, Tateho Chemical Industries, Hyogo, Japan. 2. Experimental Table 1 shows the compositions of samples used for this study. For sample A, the two ceramic powders were mixed by planetary ball milling for 8 h. Ethanol, alumina balls 5 mm in diameter (SSA999, Nikkato Corp., Tokyo, Japan) and a plastic jar were used for mixing. The mixed powder was hot pressed at 1823 K for 1 h under 30 MPa. For samples T10 and T20, a modified tape casting method was employed for aligning the whiskers. The diameters and lengths of the SiC whiskers used in this study were 1–1.4 and 10–20m, respectively, according to information supplied from the manufacturer. Details of the processing conditions and modification are described elsewhere [6,8]. Tape cast products were cut and laminated at 353 K under 50 MPa for 0.5 h. Binder burn-out was performed at 823 K for 10 h in open air, and then the sample was hot pressed at 2123 K under 30 MPa for 1 h. Tribological behavior was studied by using the ball-onreciprocating flat geometry. Fig. 1 shows a schematic diagram of the high temperature wear tester. A silicon nitride ball (NDB200, Norton Co., Northboro, MA, USA) of 6.35 mm in diameter was used against a flat alumina–SiC whisker composite sample surface which was polished with 1m diamond slurry. The normal load was 40 N and the average speed was 10 mm/s. The reciprocating stroke was 5.64 mm and the duration was 1 h. Wear tests were performed at a range of temperature between 403 and 873 K. In order to examine the effect of whisker orientation on wear rate, tests were carried out on three directions (paralFig. 1. Schematic diagram of the high temperature tester. Fig. 2. Schematic representation of the sliding direction with respect to the tape casting and lamination direction. Whiskers are well aligned according to these directions. lel, normal with tape casting direction, and normal with the lamination direction). Fig. 2 shows a schematic illustration of the sliding directions with respect to whisker orientation. The wear volume of the flat sample was obtained by wear track length multiplied cross-sectional worn area that was measured from the center portion of the groove by a surface profilometer after cleaning in a ultrasonic bath. Each wear test was repeated three times and the average values of the measurements were expressed in the results. The wear scar was examined by a scanning electron microscope equipped with an energy dispersive X-ray analyzer. 3. Results Sliding on sample A was carried out on the surface in a normal direction with hot pressing direction. Different sliding directions with respect to the whisker orientation are shown in Fig. 2. Fig. 3 shows the values of the average friction coefficient and standard deviations measured during sliding parallel with tape casting direction for the test materials at different temperatures. The friction coefficient Fig. 3. Friction coefficients between the alumina–SiC whisker composite and silicon nitride ball as a function of whisker content
D-S. Lim et al/Wear 251(2001)1452-1458 2,2x1 of sample A, which had no whisker, slightly increased with increasing test temperatures. However, the friction coeffi- cients of samples with 10 and 20 vol. whiskers decreased 1.8x1 with increasing wear test temperatures. At 873K, a decrease 1.6x10 in friction with increasing whisker content was noticeable 1.4x10 Fig 4 shows the variation of wear rate with wear test temp 1.2x1 atures and whisker contents. A sharp increase of wear even 1,0x10 at 673 K is noticeable for sample A, The wear rates of sam 8.0×10 ples with 10 and 20 vol. whiskers were slightly increased 6.0x10 with increasing test temperatures. The wear rates tested above 673 K tended to decrease with increasing whisker 2.0x10° contents T10 and T20 show different friction coefficients depend hiker Content(vol % ing on the whisker orientation and sliding direction. A 403K the lowest friction coefficient was obtained for t10 Fig. 4. Wear rate of the alumina-Sic whisker composite as a function of but higher friction for T20 in the direction normal for the whisker conten tape casting direction, as shown in Fig. 5. However, the orientation effect on the friction coefficient tested at 873K diminishes for both T10 and T20 samples. The wear rate also depends on the sliding direction with respect to the 403K Z873k 403K 1.4x1 Z873K 0.4 d8.0x10 0x10 0.0 N Whisker orientation Whisker orientation 0.7 Z873K 1,4x10 2873K 0000 hiker orientation Whisker orientation (b) Fig. 5. Effect of whisker orientation on friction coefficients of (a)sample Fig. 6. Effect of whisker orientation of wear rate of(a)sample T1O an TI0 and (b) sample T20 worn at 403 and 873K (b)sample T20 worn at 403 and 873K
1454 D.-S. Lim et al. / Wear 251 (2001) 1452–1458 Fig. 4. Wear rate of the alumina–SiC whisker composite as a function of whisker content. Fig. 5. Effect of whisker orientation on friction coefficients of (a) sample T10 and (b) sample T20 worn at 403 and 873 K. of sample A, which had no whisker, slightly increased with increasing test temperatures. However, the friction coeffi- cients of samples with 10 and 20 vol.% whiskers decreased with increasing wear test temperatures. At 873 K, a decrease in friction with increasing whisker content was noticeable. Fig. 4 shows the variation of wear rate with wear test temperatures and whisker contents. A sharp increase of wear even at 673 K is noticeable for sample A. The wear rates of samples with 10 and 20 vol.% whiskers were slightly increased with increasing test temperatures. The wear rates tested above 673 K tended to decrease with increasing whisker contents. T10 and T20 show different friction coefficients depending on the whisker orientation and sliding direction. At 403 K, the lowest friction coefficient was obtained for T10, but higher friction for T20 in the direction normal for the tape casting direction, as shown in Fig. 5. However, the orientation effect on the friction coefficient tested at 873 K diminishes for both T10 and T20 samples. The wear rate also depends on the sliding direction with respect to the Fig. 6. Effect of whisker orientation of wear rate of (a) sample T10 and (b) sample T20 worn at 403 and 873 K
D.-S. Lim et al./Wear251(2001)l452-1458 orientation of the whiskers, as shown in Fig. 6 For sample Less micro-fracture but greater formation of rolls and debris T10, the lowest wear was obtained in the direction normal compaction are shown on worn surfaces of T20 samples as with lamination direction and highest in the direction paral- compared with sample A(Fig. 7(c)and()). The rolls are lel with the tape casting direction For sample T20, wear in the most frequently observed on the wear tracks of the T20 the direction parallel to whisker orientation is higher than sample at 673 K, as shown in Fig. 8. A higher magnifica- that in the direction normal for the whisker orientation tion of the micrograph of the rolls shows smaller diameters Similar orientation effect on the wear rate is observed for and distinguishes from whiskers( Fig. 9). The SEM micro- both low and high temperatures graphs of the worn surfaces of parallel, normal with the tape The worn surfaces of sample A and sample T20 at three casting direction, and normal with the lamination direction different temperatures are shown in Fig. 7. Smoother sur- at 873 K, are shown in Fig. 10. The smoothest feature is faces are shown for both A and T20 samples tested at 403K, shown on the worn surface of the normal to the lamination as shown in Fig. 7(a) and(b), respectively. For sample A, direction. EDS analysis confirmed that the worn surface had micro-fractured regions and smeared wear debris partie a much higher silicon content compared to that of unworn temperature(Fig. 7(a), (c)and( surface as shown in Fig. 11 (b) 5 um Fig. 7. SEM micrographs of wom surfaces for (a) sample A at 403 K,(b) sample T20 at 403 K,(c) sample A at 673K,(d) sample T20 at 673 K,(e) ple A at 873K and (f) sample 120 at 873K
D.-S. Lim et al. / Wear 251 (2001) 1452–1458 1455 orientation of the whiskers, as shown in Fig. 6. For sample T10, the lowest wear was obtained in the direction normal with lamination direction and highest in the direction parallel with the tape casting direction. For sample T20, wear in the direction parallel to whisker orientation is higher than that in the direction normal for the whisker orientation. Similar orientation effect on the wear rate is observed for both low and high temperatures. The worn surfaces of sample A and sample T20 at three different temperatures are shown in Fig. 7. Smoother surfaces are shown for both A and T20 samples tested at 403 K, as shown in Fig. 7(a) and (b), respectively. For sample A, micro-fractured regions and smeared wear debris particles increased with increasing temperature (Fig. 7(a), (c) and (e)). Fig. 7. SEM micrographs of worn surfaces for (a) sample A at 403 K, (b) sample T20 at 403 K, (c) sample A at 673 K, (d) sample T20 at 673 K, (e) sample A at 873 K and (f) sample T20 at 873 K. Less micro-fracture but greater formation of rolls and debris compaction are shown on worn surfaces of T20 samples as compared with sample A (Fig. 7(c) and (f)). The rolls are the most frequently observed on the wear tracks of the T20 sample at 673 K, as shown in Fig. 8. A higher magnification of the micrograph of the rolls shows smaller diameters and distinguishes from whiskers (Fig. 9). The SEM micrographs of the worn surfaces of parallel, normal with the tape casting direction, and normal with the lamination direction at 873 K, are shown in Fig. 10. The smoothest feature is shown on the worn surface of the normal to the lamination direction. EDS analysis confirmed that the worn surface had a much higher silicon content compared to that of unworn surface as shown in Fig. 11
1456 D-S. Lim et al/Wear 251(2001)1452-1458 Fig. 8. SEM micrographs of worn surfaces at 673 K for(a) sample A, Fig. 10. SEM micrographs of worn surfaces at 873K for T20 samples (b) sample T10 and(c)sample T20 (a) sliding parallel with tape casting direction;( b) sliding normal with tape casting direction;(c)sliding normal with lamination direction. The results obtained in this study indicate that the addi- tion of whiskers has a beneficial effect on the friction and wear rate of SiC whisker reinforced alumina composites at high temperatures. Different tribological characteristics with increasing temperature were also shown for the mono- lithic alumina and the whisker reinforced alumina compos ites. These wear test results indicate that a different wear mechanism might have been involved at high temperatures for the sliding wear of silicon carbide whisker reinforced alumina composites. It has reported that the specific wear Fig. 9. Magnified view of Fig. 7(c) showing rolls on a film loss of alumina significantly decreased at the temperature
1456 D.-S. Lim et al. / Wear 251 (2001) 1452–1458 Fig. 8. SEM micrographs of worn surfaces at 673 K for (a) sample A, (b) sample T10 and (c) sample T20. Fig. 9. Magnified view of Fig. 7(c) showing rolls on a film. Fig. 10. SEM micrographs of worn surfaces at 873 K for T20 samples: (a) sliding parallel with tape casting direction; (b) sliding normal with tape casting direction; (c) sliding normal with lamination direction. 4. Discussion The results obtained in this study indicate that the addition of whiskers has a beneficial effect on the friction and wear rate of SiC whisker reinforced alumina composites at high temperatures. Different tribological characteristics with increasing temperature were also shown for the monolithic alumina and the whisker reinforced alumina composites. These wear test results indicate that a different wear mechanism might have been involved at high temperatures for the sliding wear of silicon carbide whisker reinforced alumina composites. It has reported that the specific wear loss of alumina significantly decreased at the temperature
D.-S. Lim et al./Wear251(2001)l452-1458 tribochemical layer and the plastically deformed layer in the micrographs of the damaged surface. However, evidence of 8 roll formation confirms formation of films(Fig. 9). Similar 6 Si roll formations have been found on silicon nitride. alumina these soft film and rolls has been found to lower friction and wear of sliding ceramics. EDS analysis of unworn and worn surfaces also confirm that silicon rich films formed on the worn surface. As shown in Fig. I l, more rolls ar 86420 found on the worn surface at 673 K. At 673 K, the number of rolls seems to increase with the whisker contents( Fig 8 Higher temperatures facilitate faster chemical reaction and also decreases the viscosity of the glassy surface film. If 0.00.51.01.52.02,53.0354,0455.0 the reaction layer is liquid at the sliding temperatures, roll formation would be difficult but the friction might be kept lower. Therefore. the lowest friction coefficient and the least Fig. 11. EDS analysis of(a) unwom and(b) worn area. A smooth spot roll formation at 873 K for sample T20 might be due to the of the T10 sample tested at 873K was selected for worn area analysis. lowering viscosity of the reaction layer. Yust and Allard showed that even at a moderate load and sliding speed, the above 800C due to the plastic deformation accompanied by frictional temperature for SiC whisker reinforced alumina recrystallization [8]. Tested temperature is so much lower composite could rise above the test temperature by about than brittle to plastic transition temperature that the dom- 400-500oC, based on their calculation [ 14]. This temper nant mechanism might be brittle fracture and both of the ature can greatly influence tribological performance by the modification of the reaction film ture. In case of the whisker reinforced The whiskers of samples T10 and T20 were aligned with composites, the friction coefficients were decreased and the tape casting direction. Sliding on sample T10 and was the wear rates were slightly increased. This results suggest carried out in three directions with respect to whisker ori that SiC whiskers somehow lower the friction coefficient entation, but the whiskers were well aligned Samples T10 at higher temperature Miyoshi and Buckley reported that and T20 showed different friction coefficients depending the friction coefficient of the silicon carbide decreased with on the orientation and surface of the test. For sample TI an increase in temperatures from 400 to 600c due to the at 403 K, the lowest friction coefficient was obtained in the gradual removal of the contaminants of carbon and oxygen direction normal with the tape casting direction on the tape from the surface [9]. Above 800 C, the friction coefficient surface, and highest in the direction normal with the lami- decreased rapidly with an increase of temperature due nation direction, as shown in Fig. 5. Slightly higher friction graphitization of the silicon surface. Removal of contar oefficients in the direction normal with the tape casting inants and the graphitization might contribute to lower direction were noticed for sample T20 at 403 K. However, friction coefficient of Sic reinforced alumina composites. the friction coefficients depending on different sliding di The analysis of the worn surface indicated that a tribo- rections are maintained to be approximately 0.35 and 0.27 chemical layer can be formed, as evidenced by observed at 873 K for T10 and T20, respectively. This result indicates roll debris. Roll debris has been observed for silicon-based that anisotropic effect on the friction diminishes at 873K ceramics and Sic whisker reinforced alumina composites The frictional surface must be covered in a similar manner [5, 10]. The formation of the tribochemical layer can be pro- in spite of the different whisker orientation. However, the moted from the oxidation of whiskers or reaction products effect of sliding direction on wear rate at 873 and 403K between the whisker and the matrix as suggested by Della- show similar trends. Cyfika and Hombogen also reported Corte [5]. The formation of a tribochemical layer will lower that wear resistance showed a much more pronounced the friction and wear rate as whisker contents increased, as anisotropy than friction [15] shown in Figs. 3 and 4. The monolithic alumina shows the least amount of wear at 403K. as shown in Fig 3. This low temperature result was explained by the dominant effect of 5. Conclusions the matrix grain size in a previous publication [4]. Above 673K, the formation of a tribochemical layer plays more The dominant wear and friction mechanism change with mportant role in the friction and wear of the SiC whisker increase in test temperature. The matrix grain size is the reinforced alumina composite. Evidence for this formation dominant mechanism at a relatively low temperature but the of a tribochemical layer or roll formation, is easily seen reaction layer due to the whiskers can be considered to be the on the worn surfaces of Sic whisker reinforced alumina dominant mechanism for the high temperature sliding wear composites(Figs. 7 and 8). It is difficult to distinguish the of the Sic whisker reinforced alumina composite. Above
D.-S. Lim et al. / Wear 251 (2001) 1452–1458 1457 Fig. 11. EDS analysis of (a) unworn and (b) worn area. A smooth spot of the T10 sample tested at 873 K was selected for worn area analysis. above 800◦C due to the plastic deformation accompanied by recrystallization [8]. Tested temperature is so much lower than brittle to plastic transition temperature that the dominant mechanism might be brittle fracture and both of the friction coefficients and the wear rates were increased with increasing temperature. In case of the whisker reinforced composites, the friction coefficients were decreased and the wear rates were slightly increased. This results suggest that SiC whiskers somehow lower the friction coefficient at higher temperature. Miyoshi and Buckley reported that the friction coefficient of the silicon carbide decreased with an increase in temperatures from 400 to 600◦C due to the gradual removal of the contaminants of carbon and oxygen from the surface [9]. Above 800◦C, the friction coefficient decreased rapidly with an increase of temperature due to graphitization of the silicon surface. Removal of contaminants and the graphitization might contribute to lower friction coefficient of SiC reinforced alumina composites. The analysis of the worn surface indicated that a tribochemical layer can be formed, as evidenced by observed roll debris. Roll debris has been observed for silicon-based ceramics and SiC whisker reinforced alumina composites [5,10]. The formation of the tribochemical layer can be promoted from the oxidation of whiskers or reaction products between the whisker and the matrix as suggested by DellaCorte [5]. The formation of a tribochemical layer will lower the friction and wear rate as whisker contents increased, as shown in Figs. 3 and 4. The monolithic alumina shows the least amount of wear at 403 K, as shown in Fig. 3. This low temperature result was explained by the dominant effect of the matrix grain size in a previous publication [4]. Above 673 K, the formation of a tribochemical layer plays more important role in the friction and wear of the SiC whisker reinforced alumina composite. Evidence for this formation of a tribochemical layer or roll formation, is easily seen on the worn surfaces of SiC whisker reinforced alumina composites (Figs. 7 and 8). It is difficult to distinguish the tribochemical layer and the plastically deformed layer in the micrographs of the damaged surface. However, evidence of roll formation confirms formation of films (Fig. 9). Similar roll formations have been found on silicon nitride, alumina and silicon carbide composites [11–14]. The formation of these soft film and rolls has been found to lower friction and wear of sliding ceramics. EDS analysis of unworn and worn surfaces also confirm that silicon rich films formed on the worn surface. As shown in Fig. 11, more rolls are found on the worn surface at 673 K. At 673 K, the number of rolls seems to increase with the whisker contents (Fig. 8). Higher temperatures facilitate faster chemical reaction and also decreases the viscosity of the glassy surface film. If the reaction layer is liquid at the sliding temperatures, roll formation would be difficult but the friction might be kept lower. Therefore, the lowest friction coefficient and the least roll formation at 873 K for sample T20 might be due to the lowering viscosity of the reaction layer. Yust and Allard showed that even at a moderate load and sliding speed, the frictional temperature for SiC whisker reinforced alumina composite could rise above the test temperature by about 400–500◦C, based on their calculation [14]. This temperature can greatly influence tribological performance by the modification of the reaction film. The whiskers of samples T10 and T20 were aligned with the tape casting direction. Sliding on sample T10 and was carried out in three directions with respect to whisker orientation, but the whiskers were well aligned. Samples T10 and T20 showed different friction coefficients depending on the orientation and surface of the test. For sample T10 at 403 K, the lowest friction coefficient was obtained in the direction normal with the tape casting direction on the tape surface, and highest in the direction normal with the lamination direction, as shown in Fig. 5. Slightly higher friction coefficients in the direction normal with the tape casting direction were noticed for sample T20 at 403 K. However, the friction coefficients depending on different sliding directions are maintained to be approximately 0.35 and 0.27 at 873 K for T10 and T20, respectively. This result indicates that anisotropic effect on the friction diminishes at 873 K. The frictional surface must be covered in a similar manner, in spite of the different whisker orientation. However, the effect of sliding direction on wear rate at 873 and 403 K show similar trends. Cyffka and Hornbogen also reported that wear resistance showed a much more pronounced anisotropy than friction [15]. 5. Conclusions The dominant wear and friction mechanism change with increase in test temperature. The matrix grain size is the dominant mechanism at a relatively low temperature but the reaction layer due to the whiskers can be considered to be the dominant mechanism for the high temperature sliding wear of the SiC whisker reinforced alumina composite. Above
D.~S. Lim et a./Wewr251a2001)l452-1458 673K, the wear rate and the friction coefficient linearly 44]CS. Yust, Tribological behavior of whisker reinforced decrease with increasing whisker content. The friction coef- composite materials, in: S. Jahanmir(Ed. ), Friction and Wear ficients at higher temperatures do not vary with sliding orien- Ceramics, Marcel Dekker, New York, 1994, pp. 199-224 tation Sliding normal to the whisker length direction caused 5]C. Della Corte, Tribological characteristics of silicon carbide whisker reinforced alumina at elevated temperature, in: S. Jahanmir(Ed.) less wear than that parallel to it in all test temperature range Friction and Wear of Ceramics. Marcel Dekker. New York. 1994 pp.225-259 [6] D-S. Lim, D-S Park, B -D. Han, T.-S. Kan, Tribological behavior Acknowledgement of alumina reinforced with unidirectionally oriented Sic whiskers, wear225229(1999) [7 E. Hormbogen, Des d wear of materials with hetero The authors(Korea University) would like to acknowl- es, Wear 111(4)(1986)391 edge the financial support for this study provided by the [8] T Senda, F. Yano, J. Drenan, E. Yasuda, R.C. Bradt, Brittle to ductile Korea Science and Engineering Foundation (KOSEF transition in sliding wear of alumina, Ceram. Eng. Sci. Proc. 20(3) through the Ceramic Processing Research Center(CPRC) (1999)43-450. at Han Yang University 9] K. Miyoshi, D H. Buckley, Surface chemistry and friction behavior of the silicon carbide(000 1)surface at temperatures to 1500C, NASA Technical Paper 1813, Vol 27, 1981, pp. 1-10 [10] C.S. Yust, J.T. Leitnaker, C.E. Devore, Wear of an alumina-silicon References carbide whisker composite, Wear 122(1988)151-164 [11] D-S. Park, S. Danyluk, M. McNallan, Ceram. Trans. 10(1990) IE D. Kra . F. Amateau, G L. Messing, Processin 159-180 tion of laminated Sic whisker reinforced AlO3 [12 T.E. Tomizawa, H. Fischer, Wear 105(1985)29-45 Composite Mater. 25(1991)416-432 [13R.S Gates, S.M. Hsu, E.E. Klaus, Tribol. Trans. 32(1989)357-363 2]L. Shaw, R Abbaschian, Fabrication of Sic whisker rei [14C.S. Yust, L.F. Allard, in: V.J. Tennery(Ed. ) Ceramic Materials and MoSi2 composites by tape casting, J. Am. Ceram. Soc omponents for Engines, American Ceramic Society, Westerville, 3129-3132 OH,1989,1212pp [3]M. Wu, G.L. Messing, Fabrication of oriented SiC whisker reinforced [15] M. Cyfika, E. Hombogen, Anisotropy of Friction and Wear of Fiber mullite matrix composites by tape casting, J. Am. Ceram. Soc. 77 Reinforced Epoxy-Resins, ASM International, Metals Park, OH, (1994) 1988,pp.53-59
1458 D.-S. Lim et al. / Wear 251 (2001) 1452–1458 673 K, the wear rate and the friction coefficient linearly decrease with increasing whisker content. The friction coef- ficients at higher temperatures do not vary with sliding orientation. Sliding normal to the whisker length direction caused less wear than that parallel to it in all test temperature ranges. Acknowledgements The authors (Korea University) would like to acknowledge the financial support for this study provided by the Korea Science and Engineering Foundation (KOSEF) through the Ceramic Processing Research Center (CPRC) at Han Yang University. References [1] E.D. Kragness, M.F. Amateau, G.L. Messing, Processing and characterization of laminated SiC whisker reinforced Al2O3, J. Composite Mater. 25 (1991) 416–432. [2] L. Shaw, R. Abbaschian, Fabrication of SiC whisker reinforced MoSi2 composites by tape casting, J. Am. Ceram. Soc. 78 (1995) 3129–3132. [3] M. Wu, G.L. Messing, Fabrication of oriented SiC whisker reinforced mullite matrix composites by tape casting, J. Am. Ceram. Soc. 77 (1994) 2586–2592. [4] C.S. Yust, Tribological behavior of whisker reinforced ceramic composite materials, in: S. Jahanmir (Ed.), Friction and Wear of Ceramics, Marcel Dekker, New York, 1994, pp. 199–224. [5] C. DellaCorte, Tribological characteristics of silicon carbide whisker reinforced alumina at elevated temperature, in: S. Jahanmir (Ed.), Friction and Wear of Ceramics, Marcel Dekker, New York, 1994, pp. 225–259. [6] D.-S. Lim, D.-S. Park, B.-D. Han, T.-S. Kan, Tribological behavior of alumina reinforced with unidirectionally oriented SiC whiskers, Wear 225229 (1999) 868–873. [7] E. Hornbogen, Description and wear of materials with heterogeneous and anisotropic microstructures, Wear 111 (4) (1986) 391–402. [8] T. Senda, F. Yano, J. Drenan, E. Yasuda, R.C. Bradt, Brittle to ductile transition in sliding wear of alumina, Ceram. Eng. Sci. Proc. 20 (3) (1999) 43–450. [9] K. Miyoshi, D.H. Buckley, Surface chemistry and friction behavior of the silicon carbide (0 0 0 1) surface at temperatures to 1500◦C, NASA Technical Paper 1813, Vol. 27, 1981, pp. 1–10. [10] C.S. Yust, J.T. Leitnaker, C.E. Devore, Wear of an alumina–silicon carbide whisker composite, Wear 122 (1988) 151–164. [11] D.-S. Park, S. Danyluk, M. McNallan, Ceram. Trans. 10 (1990) 159–180. [12] T.E. Tomizawa, H. Fischer, Wear 105 (1985) 29–45. [13] R.S. Gates, S.M. Hsu, E.E. Klaus, Tribol. Trans. 32 (1989) 357–363. [14] C.S. Yust, L.F. Allard, in: V.J. Tennery (Ed.), Ceramic Materials and Components for Engines, American Ceramic Society, Westerville, OH, 1989, 1212 pp. [15] M. Cyffka, E. Hornbogen, Anisotropy of Friction and Wear of Fiber Reinforced Epoxy-Resins, ASM International, Metals Park, OH, 1988, pp. 53–59
